The present invention relates to the spreading (wetting) of aqueous solutions on water-repellent (hydrophobic) surfaces. In particular, the present invention relates to combinations of surfactants which provide improved spreading of aqueous solutions on solid hydrophobic surfaces.
Pure water does not spread on hydrophobic surfaces. When a drop of water comes in contact with such a surface, it forms a more or less spherical droplet that makes a large contact angle (e.g., 90° or more, measured in the water) with the surface. The addition of a surfactant to the water decreases the contact angle that the water makes with the surface, and causes the water to spread over the surface. For a droplet of fixed volume, the smaller the contact angle that the aqueous solution makes with the surface, the larger will be the area that the solution will spread over the surface.
The decrease in the contact angle and consequent increase in the area spread by the aqueous solution over the hydrophobic substrate depends upon the nature and concentration of the added surfactant. Some surfactants, at the concentrations generally used for this purpose (0.1% or less in the aqueous phase), are very effective at increasing the spreading area, while others are not. Surfactants whose molecular structure contains a hydrophilic group that carries no formal charge are often used for improving the spreading of aqueous media on hydrophobic substrates. The absence of a formal charge on such nonionic surfactant molecules makes them less sensitive to electrolytes and other charged components of the aqueous formulations.
The enhancement of spreading or wetting of aqueous solutions on solid water-repellent surfaces is an important “performance property” of surfactants. The wetting of solid surfaces is important in many industrial and consumer processes such as: 1) the cleaning of greasy industrial equipment (machinery, automotive parts), 2) the coating of or printing on water-repellent surfaces, such as polyethylene film, 3) the spreading of herbicides and pesticides on plant leaves (which are generally water repellent), 4) the shampooing of hair coated with body oils, and 5) the cleaning of greasy consumer items (e.g., cooking equipment).
When the surface is highly water repellent (e.g., polyethylene, waxy leaves), many surfactants are not suitable for this purpose, since their aqueous solutions do not spread well. Examples of such surfactants include those that contain hydrocarbon chains.
Trisiloxane-based surfactants are excellent wetting agents for hydrophobic surfaces. Their aqueous solutions spread over much greater areas than do surfactants with hydrocarbon chain hydrophobic groups. Accordingly, surfactants with trisiloxane hydrophobic groups are often called “superspreaders” or “superwetters.” However, these trisiloxane-based surfactants are much more expensive than hydrocarbon chain surfactants. In addition, trisiloxane-based surfactants are unstable in aqueous solution unless the pH of the solution is neutral. Therefore, there is a need for less expensive surfactants and surfactants which can be used over a wide pH range.